Hydraulic or pneumatic cylinder

The present invention relates to pneumatic and hydraulic cylinders.

Hydraulic and pneumatic cylinders suffer from the disadvantage that should there be any leakage, the cylinder will either extend or retract depending on its preloaded condition.

It is the object of the present invention to overcome or substantially ameliorate the above disadvantage.

There is disclosed herein a hydraulic or pneumatic cylinder including:

a bore, the bore generally surrounding a chamber having opposite ends;

a piston in said bore and co-operating therewith to divide said chamber into a first and a second sub-chamber;

a piston rod fixed to and extending from said piston and extending beyond one of said ends;

means closing said opposite ends;

releasable locking means to retain said piston at a desired location within said bore, said locking means being released to permit movement of said piston upon delivery of a fluid under pressure to one of said chambers; and

duct means to provide for fluid flow from said second sub-chamber to said first sub-chamber when said locking means is engaged with said piston to retain said piston at said desired location.

Preferably, the locking means includes a pawl mounted in said body, and a recess in said piston, said recess being alignable with said pawl so as to be engaged therewith to retain the piston at said desired location, said pawl being movable between a first position at which it is engaged within said recess and a second position spaced from said recess to provide for movement of said piston.

Preferably, the cylinder includes a spring urging the pawl into engagement with said recess.

Preferably, the delivery of fluid under pressure to said first sub-chamber causes movement of said pawl from the first position to the second position thereof.

A preferred form of the present invention will now be described by way of example with reference to the accompanying drawing which schematically depicts in section side elevation a pneumatic or hydraulic cylinder.

In the accompanying drawing there is schematically depicted a hydraulic or pneumatic cylinder 10 . The cylinder 10 has a body 11 providing a bore 12 generally surrounding a chamber 13 . The chamber 13 has opposite ends 14 and 15 which are closed by plug members 16 and 17 .

Mounted within the body 11 and slidably engaging the cylindrical surface of the bore 12 is a piston 18 . The piston 18 divides the chamber 13 into a first sub-chamber 19 and a second sub-chamber 20 . The piston 18 has a tapered leading surface 21 which is of frusto-conical configuration. The piston is further provided with an annular recess 22 as well as a wear ring 23 and seal 24 .

The body 11 includes a housing 25 providing a bore 39 which slidably receiving and guiding a pawl 26 . The pawl 26 has a leading projection 27 which is aligned with an aperture 28 in the body 11 . A spring 29 urges the pawl 26 to a position at which the projection 27 is located within the chamber 13 .

Located between the pawl 26 and an end cap 30 is a locking chamber 31 within which the spring 29 is located. Extending from the chamber 31 is a duct 32 extending to an aperture 33 . The aperture 33 communicates with the sub-chamber 20 when the piston 18 is located adjacent the end 15 .

The piston 18 is sized so that there is an annular space 34 permitting fluid to pass from the sub-chamber 19 to a position at which pressure can be applied to the pawl 26 .

Extending from the body 11 is a first fluid coupling 36 which provides for movement of fluid relative to the sub-chamber 19 while a second fluid coupling 35 provides for movement of fluid relative to the second sub-chamber 20 .

Attached to and extending from the piston 18 is a piston rod 37 which exits via the plug member 16 which sealingly engages the longitudinal peripheral surface of the piston rod 37 .

In operation of the above described hydraulic cylinder 10 , fluid under pressure is delivered to the sub-chamber 19 . Fluid passing the passage 34 and entering the chamber 40 causes retraction of the pawl 26 to a position at which the projection 27 is no longer engaged within the recess 22 . Pressure within the sub-chamber 19 causes movement of the piston 18 toward the end 14 . Fluid is allowed to leave the sub-chamber 20 via the fluid coupling 35 . When the pawl 26 moves to its retracted position, fluid within the chamber 31 is allowed to return to the chamber 13 via the duct 32 . When the piston 18 is spaced from the end 17 and fluid under pressure is delivered to the sub-chamber 20 , the piston 18 moves towards the end 15 and engages the projection 27 . The tapered surface 21 engages the projection 27 and causes the pawl 26 to move to a position at which the piston 18 can pass. Once the piston 18 is in the position depicted, the pawl 26 moves to a position at which the projection 27 is engaged within the recess 22 . Fluid from the sub-chamber 20 passes through the duct 32 to urge the pawl 26 to engage within the recess 22 . This movement is enhanced by means of the spring 29 .

The above mentioned duct 32 also enables the cylinder 10 to be used in a phasing circuit. In relation to the cylinder 10 being used in a phasing circuit, the pawl 26 is provided with a single acting seal 38 which permits the flow of fluid only from the chamber 31 past the pawl 26 to enter the sub-chamber 19 . When the cylinder 10 is used in a phasing circuit and the piston 18 located adjacent the end 15 , fluid can flow via the duct 32 past the seal 38 to exit via the coupling 36 from where it would pass to the next cylinder. Reverse flow is prevented by the seal 38 .

The above preferred embodiment ensures that the piston 18 is retained adjacent the end 15 until fluid under pressure is delivered to the sub-chamber 19 .